A carburetor body that has been reworked by Trock Cycle. The shop bored out the throat to 42 mm and blended the venturi. A new, larger throttle butterfly will be installed. They installed a new fuel inlet that has a larger capacity and rotates. It takes high-precision machines to do this work.
The Dakota Kid bores the carburetor's throat to 42mm, recontoured the slide, and polished every part that encounters the airflow. It also features a larger fuel inlet and a custom jet needle.
To increase its fuel capacity, Wood Performance Products installed a spacer (arrow) between the carburetor body and the float bowl. They bore it out to 42 mm, recontoured the slide, and installed a custom fuel inlet with a larger needle and seat.
All of the three carburetors had a bleed hole (arrow) in the throttle butterfly. This hole allows the carburetor to be run on motors that have high-performance camshafts with more valve overlap. These cams require more air past the butterfly at idle than stock cams. If the butterfly is opened for this extra air with the adjustment screw, then the off-idle transfer ports will be uncovered, which will cause idle and hesitation problems.
This CV carburetor tuner kit from DynoJet has all of the parts needed to recalibrate a CV carburetor for performance riding. No special tools are needed, and the instructions have comprehensive tuning tips.
The Yost Power Tube kit features a special dual chamber emulsion tube that refines the fuel and air coming out of the main jet. The smaller droplets of fuel make for an air-fuel mixture that burns more completely, delivering more power out of the same amount of gas.
The emulsion tube (A) is threaded into the bottom of the carburetor body. The main jet (B) is screwed into the end of the emulsion tube.
The slow jet is screwed into the carburetor body in the bottom of a hollow well (arrow). This jet controls the fuel flow as much as 1/4-throttle.
The slide moves up into the carburetor body as vacuum is applied to a hole (arrow) at the bottom rear of the slide. Enlarging this hole causes the slide to move up faster. Both of the tuning kits included drill bits for this operation. One kit included a #29 bit, the other a 1/8-inch bit.
The jet needle fits inside of the slide. It is inserted from the top and retained by the plastic spring seat (arrow). Both of the kits included custom adjustable needles. This is where the patience comes in; if the needle has to be adjusted, then the carburetor must be removed to do it. After about the third time, it becomes easy.
The Da Vinci carburetor resembles a CV from the outside. However, it is a straight butterfly carburetor that has been made out of the CV body.
A machined aluminum plug has been installed in place of the slide assembly. The plug fits all the way into the grooves that the slide moved in.
This booster venturi is installed into the carburetor. It is secured with a special emulsion tube. The fuel flows up the emulsion tube into the booster venturi and out through the small holes in the center into the air entering the carburetor.
The carburetor's throat is very smooth and has been contoured for maximum efficiency. The designs used in this carburetor were worked out in the special carburetors that Da Vinci builds for automotive circle-track racing.
More Harleys use the Keihin CV (constant velocity) carburetor than any other type. Harley-Davidson introduced this carburetor on '89 Sportsters and '90 Big Twins; they have sold almost 2 million -- with a capital M -- CV-equipped bikes since then. In stock form, it is a decent carburetor; it has a 40mm throttle bore and a 39mm venturi. Early California models had an off-idle hesitation; this was caused by a lean, slow-speed jet, but the problem was cured with the introduction of a #45 slow jet.
An '02 Twin Cam bike with a stock CV carburetor develops 60 to 63 hp. With a free-flowing air filter, performance mufflers, and installing a larger main jet in the carburetor, the Twin Cam motor will produce 70 to 75 hp. However, there are more ponies inside the CV carburetor.
On a hopped-up Evo or a Twin-Cam motor (high compression, performance camshaft, ignition, air filter, and mufflers), a stock CV carburetor becomes the limiting factor in the motor's output. There are several ways to improve the potential of the CV carburetor: buy a custom-modified carburetor, have yours modified by one of the carburetor shops, or modify it yourself.
The difference between having the work done and doing it yourself is in the amount of modification that can be accomplished. To find out the latest trends in hot CV carburetors, we picked up some samples of re-worked carburetors and a couple of CV carburetor hop-up kits. The carburetors all showed a high level of workmanship, and the kits were well thought out.
Professionally Reworked Carburetors
The shops who work a lot with the CV carburetor are boring out the throttle body to 42 mm, installing a larger butterfly, recontouring the slide, and drilling bleed holes in the butterfly. These carburetors have the highest potential output; most of these operations are beyond the abilities of most bikers. The amount of power between a professionally modified carburetor and a re-jetted stock CV carburetor can be as much as 20 hp. The cost is between $275 and $500, depending on how much work is done to the carburetor, which could be estimated at $25 per horsepower.
When Is a CV No Longer a CV Carburetor?Da Vinci Performance Products converts a Keihin CV carburetor into a straight butterfly carb. Da Vinci does this by removing the slide assembly, installing a machined plug, boring out the throat to 42 mm, enlarging the venturi to 41 mm, and then fitting a booster venturi into the carburetor's throat. That is the simple explanation; the actual work is very extensive. The reasoning behind all of this work is that without the jet needle sitting perpendicular to the butterfly, there will be less turbulence in the air-fuel mixture as it enters the intake tract. We can attest that this carburetor produced 12.6 hp more than a stock re-jetted CV carburetor on a hopped-up 1200 Sportster (reworked heads, pop-up pistons, performance camshafts, ignition, air filter, and mufflers).
IMAGE GROUP ABOVEReworking Your CV CarburetorBoth of the two CV carburetor tuning kits -- DynoJet and Yost Power Tube -- we checked out included jets, jet needle, emulsion tube, drill bits, and complete instructions. Modifying a CV carburetor requires a bit of skill and a lot of patience; installing the parts is just the beginning. You must then fine-tune the carburetor to match the bike's motor and your riding style. You will find that the cost of the kit (about $80) and labor are well worth the effort. The modified carburetor will not only add more power to the motor, but will make the motor perform better in low to middle rpm ranges.
IMAGE GROUP BELOWWhen Is a CV No Longer a CV Carburetor?Da Vinci Performance Products converts a Keihin CV carburetor into a straight butterfly carb. Da Vinci does this by removing the slide assembly, installing a machined plug, boring out the throat to 42 mm, enlarging the venturi to 41 mm, and then fitting a booster venturi into the carburetor's throat. That is the simple explanation; the actual work is very extensive. The reasoning behind all of this work is that without the jet needle sitting perpendicular to the butterfly, there will be less turbulence in the air-fuel mixture as it enters the intake tract. We can attest that this carburetor produced 12.6 hp more than a stock re-jetted CV carburetor on a hopped-up 1200 Sportster (reworked heads, pop-up pistons, performance camshafts, ignition, air filter, and mufflers).